When preparing silica gel it is common to use a structure directing agent in the form of a cationic surfactant and a ripening control substance such as N,N-dimethyl formamide. The problem of using a cationic surfactant is that it acts both as emulsifier and as structure template, thereby making it difficult to control the pore size while controlling particle size. The present invention describes a method for preparing spherical silica particles without the use of a cationic surfactant and the toxic substance N,N-dimethyl formamide.
The present invention is based on further development of the method for preparation of silica gel in “Development of standard operation procedures for the manufacture of n-octadecyl bonded silicas as packing material in certified reference columns for reversed-phase liquid chromatography” by C. du Fresne von Hohenesche, V. Ehwald, K. K. Unger, J. Chromatogr. A 1025 (2004) 177-187.
WO 2003/102001 A1 discloses polyol-modified silanes as precursors for silica. Ikai et al., Chem. Asian J. 2008, 3, 1494-1499, and EP 1 887 354A1 disclose beads for optical-isomer resolution and a process for producing the same. EP 2 031 388 A1 discloses fillers for optical isomer separation. US 2010/0221542 discloses a process for preparing a porous silica particle using a water-soluble pore-forming agent.
When preparing the silica particles it is necessary to ensure that the pore size and pore volume is controlled, as these parameters have a great impact on the usability of the silica gel, especially when the particles are used as chromatographic media. The pore structure is traditionally controlled by adding drying control agents such as the toxic N,N-dimethyl formamide. Also the particle size distribution in the emulsion is important, as this determines the yield of the process. This is because most in most applications of macroporous silica particles only a certain particle size fraction can be used. Typically the desired particle size fraction is obtained by sieving or cyclone fractionation. Moreover, it is important to prepare mechanically robust particles, in particular for the purpose of HPLC applications, and thereby suppress the breakage of particles in the preparation process and upon use.
Hence, there is a need for a better process for preparing macroporous silica particles, said process reducing or eliminating the need for toxic agents and increasing the yield and quality of the final sieved product.